A wide variety of hydraulic engine mount assemblies are presently available to isolate vibrations from a vehicle engine, transmission or the like and the vehicle frame. A very popular mount in use today is the hydraulic mount disclosed in U.S. Pat. No. 4,588,173 to Gold et al.
The hydraulic mount assembly of this prior invention includes a reinforced hollow elastomeric body which is closed on one end by a resilient diaphragm so as to form a cavity. This cavity is partitioned by a rigid plate into two chambers which are in fluid communication through a large decoupling orifice and an orifice track, both within the partition plate.
During operation, small amplitude vibrations produce no damping due to the decoupling orifice. Larger amplitude vibrations are hydraulically dampened by the flow of fluid along the orifice track between the two chambers. This mount assembly is adopted primarily for engine mounting where a relatively high degree of stiffness is desired and space is generally not at a premium.
Another hydraulic mount of fairly recent origin is disclosed by U.S. Pat. No. 4,262,886 to LeSalver et al. assigned to Peugeot. This patent discloses an integral hydraulic engine mount including upper and lower base mounting plates. An elastomeric body is interposed between the two plates. The elastomeric body is hollow and contains at least three hydraulic chambers; a relatively large primary or core chamber and two smaller secondary or peripheral chambers. The outer wall sections of the two peripheral chambers are relatively thin, and thus flexible, to allow radially outward deflection. The base plates span substantially the full area of the core chamber. The operational stiffness of this mount is generally comparable to the engine mount of the Gold patent.
The three chambers in the Peugeot patent are in fluid communication with one another through orifice tracks located within the elastomeric body. The elastomeric annular wall of the body surrounding the core chamber is thick to resist radial deflection and provide the necessary compressive support. The resistance to radial deflection of the elastomeric walls is increased somewhat by the addition of reinforcing rings molded within the elastomer.
During operation of this type of integral mount, the vibrational forces encountered tend to compress the mount, and accordingly the primary chamber. Due to the compressive pressure, some of the fluid is forced along the two damping orifice tracks, while some of the force tends to directly deform the annular wall of the mount. The outside wall sections surrounding the secondary or peripheral chambers are thinner to allow limited outward expansion. However, because the core chamber has a cross sectional area only as large as the base plate(s), a large proportion of the flexing force is transmitted directly to the relatively thick annular wall. This action tends to provide an overly stiff damping characteristic to the mount. Also, the peripheral chambers are not rigidly separated from the primary chamber and therefore, some of the internal forces of the fluid in the core chamber act radially outwardly, thereby lessening the damping effect provided by the mount.
A need thus exists for a hydraulic mount providing full hydraulic damping and improved performance with a softer damping characteristic during operation. Such a mount would have simple integral construction providing a greater proportion of fluid damping and less direct mechanical flexing of the support walls. The improved fluid action provides enhanced operating characteristics that span throughout the entire range of vehicle operating conditions.